Recent advances in video compression technology have spurred significant interest in the are of high definition television (HDTV) involving digital broadcast. As a result, much literature each been published regarding same. For example, in the paper by W. Schreiber, "Considerations in the Design of HDTV Systems for Terrestrial Broadcasting," Electronic Imaging, October 1990, it is pointed out that if the digital broadcast is implemented with a single resolution scheme as in the present terrestrial broadcast, the broadcast bandwidth would be underutilized for areas close to a signal transmitter. This stems from the fact that the power of a transmitted signal appearing at a receiver decreases with its distance from the transmitter, while the noise corrupting the signal remains virtually unchanged with the distance. As such, receivers closer to the transmitter are provided with signals of a higher carrier-to-noise ratio (CNR) (which is, as is well-known, a measure of the relative average power of a carrier signal to its accompanying noise) and thus a lower probability of recovery errors. Therefore, in providing the same HDTV quality within a coverage area, the required bandwidth for the receivers close to the transmitter need not be as much as for those relatively far therefrom.
Another problem with the single resolution scheme coupled with the digital technology is its failure to provide a moderate degradation beyond the intended coverage area. That is, due to design limitations, a signal transmitted from a digital, single resolution system would normally have too low a CNR for a receiver beyond the intended coverage area to recover even a degraded signal.
Attempts have been made to solve the aboveidentified problems with the single resolution scheme. One such attempt involves the use of a multiresolution (MR) scheme as proposed in: "Digital Spectrum Compatible HDTV System," AT&T Zenith Technical Report, September 1991. In accordance with this proposal, two video signals of different bit-rates ar transmitted for two different coverage areas in a time division multiplexed manner. The signal of a higher bit rate offering a relatively fine resolution is intended for receivers close to the transmitter in a first coverage area, and the signal of the relatively low bit rate offering relatively coarse resolution is intended for receivers in the second, farther coverage area, thereby achieving a moderate transition of TV quality between the first and second coverage areas.
Another MR scheme, which is proposed in: "Advanced Digital Television: System Description," Sarnoff/NBC/Philips/Thomson Technical Report, February, 1991, calls for bit prioritization in its coding scheme and frequency division multiplexed transmission. However, as pointed out in: T. Cover, "Broadcast Channels," IEEE Transactions on Information Theory, vol. IT-18, pp. 2-14, January 1972, the multiplexed schemes, be it frequency division or time division, are less desirable in terms of bandwidth efficiency for communicating different information simultaneously to different receivers, compared with an embedded scheme where a signal containing the composite information to be decoded by different receivers.
Still another MR scheme focusing on spectral efficiency, which is described in: W. Schreiber, "All-digital HDTV Terrestrial Broadcasting in the U.S.: Some Problems and Possible Solutions," Workshop on Advanced Television, ENST, Paris, May 1991, calls for hybrid analog-over-digital transmission. The latter scheme is however not completely digital and therefore cannot take full advantage of the digital compression technology.